Isoolefin-diolefin production process and apparatus therefore

ABSTRACT

In an isoolefin-diolefin rubber production process (e.g. a butyl rubber production process), the cold rubber slurry produced in the reaction vessel is transported from the reaction vessel to the flash tank during which time the cold slurry may be expressed to separate at least part of the cold liquid reaction medium from the isoolefin-diolefin rubber. The separated cold liquid reaction medium is transported off stream, for example by a mechanical filter, where it can be recycled back into the reaction vessel and/or where it can be used to cool a feed stream or streams of the reaction components. The isoolefin-diolefin rubber and the residual liquid reaction medium are transported to a flash tank for further processing. The ability to separate and recycle cold liquid reaction medium makes the process more economical. In one aspect, the slurry may be transported from the reaction vessel to the flash tank by a self-cleaning fully intermeshing co-rotating twin screw extruder, which helps overcome the problem of plugging due to rubber sticking to the surfaces of the slurry transfer apparatus.

FIELD OF THE INVENTION

This present invention relates to isoolefin-diolefin rubber processesand apparatuses therefor. Further, the present invention relates toprocesses and apparatuses for producing an isoolefin-diolefin rubber, toprocesses and apparatuses for separating isoolefin-diolefin rubber fromrubber slurry, and to processes and apparatuses for discharging and/ortransferring rubber slurry from an isoolefin-diolefin rubber reactor.

BACKGROUND OF THE INVENTION

Isoolefin-diolefin rubbers, for example, butyl rubber and the halobutylrubbers derived from butyl rubber (e.g. chlorobutyl rubber andbromobutyl rubber), are used extensively in a number of applications,for example in tire manufacture. Commercially, isoolefin-diolefin rubberis typically prepared by suspension copolymerization of an isoolefin(e.g. isobutylene) with a diolefin (e.g. isoprene) in the presence of acatalyst (e.g. a Friedal-Crafts type catalyst) in a liquid non-aqueousreaction medium (e.g. comprising a diluent such as an alkyl halide) at atemperature below −90° C. in a continuous reactor to form a suspensionor slurry of fine rubber “solids” in the reaction medium. The reactiontemperature may be higher with the use of specialized catalysts.

A commercial apparatus comprises, among other elements, a reactionvessel in which the reaction is continuously conducted, a flash tank forevaporating liquid reaction medium and other volatile components awayfrom the rubber which was formed in the reaction, and a discharge linefor transferring rubber slurry from the reaction vessel to the flashtank. A flash tank is normally a vessel containing heated, agitatedwater into which the rubber is discharged, forming a crumb that issuspended in water. Steam is sparged into the flash tank to maintain thetemperature and drive off volatile residual reaction medium. Inconventional processes, the flash tank is normally followed by one ormore stripping vessels to further remove residuals down to an acceptablelevel. Reaction components are normally charged to the reaction vesselin two separate feeds, a mixed feed containing monomers together with adiluent and a catalyst feed containing catalyst.

One problem encountered with such a process is the accumulation ofrubber on the inner surfaces of the reaction vessel and discharge line,which can lead to fouling and considerable down time to correct thefouling. Such accumulation is a result of the “stickiness” of theisoolefin-diolefin rubbers such as butyl rubber. In order to reduce thisproblem, relatively high flow velocities are maintained in the reactionvessel and the discharge line. The discharge line is also steam jacketedand this is believed to lead to the formation of a vapor layer on theinside surface of the discharge line that acts like a lubricating film.

In addition, it is desirable to increase the overall energy efficiencyof current commercial apparatuses. Currently, cold rubber slurry exitingthe reaction vessel is transferred directly to a flash tank by thedischarge line. In the flash tank, the liquid reaction medium includingvolatile components such as unreacted monomers, etc. is vaporized. Inorder to recycle the vaporized components, part of the process involvescondensing those vapors and cooling the liquid back to the reactiontemperature. This requires significant expenditure of energy. To reducethe amount of energy expended in cooling virgin and/or recycled reactioncomponents, it would be desirable to capture the energy associated withthe cold liquid reaction medium discharged from the reaction vessel.

WO 93/21241 to Bruzzone et al. published Oct. 28, 1993 describes aprocess for butyl rubber production. Polymerization of isobutylene withisoprene is conducted at a relatively high temperature of −50° C. in thepresence of special catalysts. The reactor comprises a verticalcylindrical domed reaction vessel, a vertical single screw extruder(i.e. a discharge screw) associated with the reaction vessel and ahorizontal double screw volatilizer connected to the discharge screw.The vertical extruder conveys and presses the solid polymer contained inthe slurry upward and “squeezes” the liquid out of the slurry allowingit to drain downward (i.e. backward) toward the reaction vessel. In thisway, valuable reaction medium including unreacted monomers at thereaction temperature is returned to the reaction vessel for furtherutilization. Residual reaction medium including residual monomers isvolatilized in the horizontal screw volatilizer and fed as a gas to aheat exchanger for cooling down to polymerization temperature beforere-entering the reaction vessel as a liquid.

While the process of WO 93/21241 permits recycling of some of thereaction medium by forcing it back into the reaction vessel, it suffersfrom several drawbacks. Firstly, the discharge screw is a single screwextruder, which is prone to fouling and is therefore unlikely tocontinue to operate for a worthwhile length of time. Secondly, theliquid reaction medium is forced back into the reaction vessel by theaction of gravity without first being taken off stream, thereby notproviding the opportunity to purify the liquid reaction medium, or theopportunity to use cold reaction medium for other cooling purposes ifdirect return of the reaction medium to the reaction vessel affects thepolymerization process. Having no opportunity to purify the liquidreaction medium may result in a critical build-up of impurities, whichwould likely result in reduced catalyst efficiency and/or low molecularweight polymer being produced. The latter may cause fouling andconsequently down time for cleaning the reactor. The inability toutilize the cold liquid reaction medium for other cooling purposesreduces flexibility of choice of energy recovery process. Thirdly, theuse of a screw devolatilizer to volatilize residual reaction medium andmonomers is not cost effective in comparison to the use of a flash tank.

GB 589,045 (the '045 patent) to Standard Oil Development Company issuedon Jun. 10, 1947 describes a process for the low temperaturepolymerization of olefins. The '045 patent indicates that cold slurryfrom a reaction vessel is transported to a vibrating screen to undergo astraining or filtering operation. It further indicates that therecovered cold liquid is then recycled back into the reaction zone.Residual reaction medium and the rubber formed in the reaction aretransported from the vibrating screen to a flash drum where unreactedmonomers and residual reaction medium are vaporized.

In order to prevent steam from the flash tank from entering thevibrating screen and thereby contaminating the reaction medium, the '045patent teaches a complicated system whereby a stream of sealing gasprovides a positive pressure from the vibrating screen into the flashtank. Such a system is impractical because the sealing gas will mix withoverhead vapors from the flash tank. The subsequent separation stepwould add significant cost to the process. Furthermore, the '045 patentteaches that the vibrating screen must be cooled to the reactiontemperature, which would be very difficult to achieve in practice.Furthermore, it has been the experience in the butyl rubber art that thebutyl rubber “solid” produced in the chemical reaction is in a fine andrelatively soft particulate form that is prone to agglomeration.Furthermore, it has been the experience in the butyl rubber art thatbutyl rubber is “sticky” even at reaction temperatures, in contrast tostatements made in the '045 patent. Therefore, one skilled in the artwould expect there to be a major problem with fouling of the vibratingscreen. Thus, a vibrating screen would not be expected to satisfactorilyseparate butyl rubber from the reaction medium. Indeed, the screening orfiltering apparatus and process described in the '045 patent are not inuse in any form today in the butyl rubber industry, almost 60 yearsafter the filing of the '045 patent, an indication of the impracticalityand general lack of usefulness of the technology described in the '045patent. Finally, the '045 patent teaches that the cold rubber slurryproduced in the reaction vessel contains from 1 to 10% by weight ofrubber. Currently, butyl rubber reactors are typically operated toproduce rubber slurries having rubber content in the region of about 25%by weight. The aforementioned problems with using a vibrating screen,particularly the fouling problem, would be exacerbated in apparatuseswhere the reactor produces rubber slurries having a rubber content ofgreater than 10% by weight.

U.S. Pat. No. 4,714,747 (the '747 patent) to Bruzzone issued on Dec. 22,1987 describes a process for the manufacture of butyl rubber. The butylrubber reaction itself is conducted in a self-cleaning twin screwextruder. The rubber slurry produced in the reaction extruder istransported to a vertical discharge screw which forces liquid reactionmedium out of the slurry and allows it to drain back into the reactionextruder while permitting gaseous monomer-solvent mixture to vent outthrough a vapor outlet line at the top of the discharge screw. Therubber phase enters a heated twin screw desolventizer at the bottom ofthe discharge screw.

The process and apparatus described in the '747 patent has severaldrawbacks. Firstly, the process relies on evaporative cooling of thereaction medium to remove the heat of polymerization. It is thereforeonly applicable to higher temperature (i.e. −20 to +150° C.)polymerization of butyl rubber, which will only work if suitable hightemperature catalysts are available. Conventional reaction temperaturesare far too low for evaporative cooling to work with normal reactionmedia and conventional catalysts will not produce acceptable molecularweight polymer at higher temperatures. Secondly, the liquid reactionmedium flows back into the reaction extruder by the action of gravitywithout first being taken off stream, thereby not providing theopportunity to purify the liquid reaction medium. Having no opportunityto purify the liquid reaction medium may result in a critical build-upof impurities, which would likely result in reduced catalyst efficiencyand/or low molecular weight polymer being produced. Thirdly, the use ofthe discharge screw and a screw devolatilizer to volatilize residualreaction medium and monomers is inefficient in comparison to the use ofa flash tank. Fourthly, the reaction is conducted in a twin screwextruder rather than a typical butyl rubber reaction vessel. The volumecapacity of such a reaction extruder must be large in order toefficiently accommodate the reaction components. A screw extruder ofsuch size is capital intensive thereby raising the cost of the apparatusconsiderably.

In the paper entitled “Extruder Isolation of Polymers and Elastomersfrom Latex Emulsions”, by Carl Hagberg presented at the InternationalLatex Conference on Jul. 22, 1998, there is described a room temperaturesystem for isolating solid polymers from latex emulsions. The systememploys a counter-rotating, non-intermeshing twin screw extruder tocontinuously wash, de-water and dry latex particles from a latexemulsion. The system further comprises one or more mechanical filterscomprising counter-rotating, fully-intermeshing twin screw extruders forremoving water from the stream.

Hagberg's apparatus and process is suited for the isolation of latexparticles from a latex/water emulsion at ambient temperatures, acompletely different art than isoolefin-diolefin suspensionpolymerizations. Isoolefin-diolefin suspension polymerization isconducted at low temperature in a non-aqueous medium in conjunction witha flash tank to remove reaction medium, whereas Hagberg's latex processis conducted at much higher temperature in an aqueous medium without theuse of a flash tank. Hagberg's non-intermeshing twin screw extruderdesign is suitable for conveying rubber in the latex system, but not forconveying rubber in the isoolefin-diolefin system, in part due to themore extreme fouling problem in isoolefin-diolefin systems.

GB 561,324 (the '324 patent) to Standard Oil and Development Companyissued on May 15, 1944 describes a low temperature polymerizationprocess for the manufacture of butyl rubber. Polymerization product issubjected to kneading as it is formed and conveyed to an extruder.Reaction medium is volatilized mainly in the kneaders. The rubber thenpasses to the extruder where any remaining reaction medium includingresidual monomers is removed as a vapor. Reaction medium is recycled asa vapor, which requires cooling before it enters the reaction vessel.This process also makes use of evaporative cooling but a low boilingcomponent (ethylene) is added to give the desired low operatingtemperature.

SUMMARY OF THE INVENTION

There is provided a process for producing an isoolefin-diolefin rubberincluding: reacting an isoolefin with a diolefin in a liquid non-aqueousreaction medium in a reaction vessel at a temperature in a range of from−110° C. to −50° C. to produce an isoolefin-diolefin rubber in a slurry;expressing the slurry in a separation zone to separate at least part ofthe liquid non-aqueous reaction medium from the isoolefin-diolefinrubber; transporting the separated liquid non-aqueous reaction mediumoff stream; and, transporting residual slurry to a slurry processingzone.

There is further provided an apparatus for producing anisoolefin-diolefin rubber including: a reaction vessel for preparing aslurry of an isoolefin-diolefin rubber in a liquid non-aqueous reactionmedium at a temperature in a range of from −110° C. to −50° C.; anexpression separator in fluid communication with the reaction vessel forseparating at least part of the liquid non-aqueous reaction medium fromthe rubber; a slurry processing vessel in fluid communication with theexpression separator for receiving residual slurry; and, a transportmeans in fluid communication with the expression separator at a pointbetween the reaction vessel and the slurry processing vessel fortransporting the separated liquid non-aqueous reaction medium offstream.

There is yet further provided an apparatus for producing anisoolefin-diolefin rubber including: a reaction vessel for preparing anisoolefin-diolefin rubber in a liquid non-aqueous reaction medium at atemperature in a range of from −110° C. to −50° C.; and, a self-cleaningfully intermeshing twin screw conveyor in fluid communication with thereaction vessel for transferring the rubber from the reaction vessel toa slurry processing vessel.

There is still yet further provided a discharge means for transferringan isoolefin-diolefin rubber slurry having a temperature in a range offrom −110° C. to −50° C. from a reaction vessel to a slurry processingvessel including: a barrel having a first end and a second end; an inletproximal the first end for receiving the rubber into the barrel; anoutlet proximal the second end for discharging the rubber from thebarrel; a set of fully intermeshing, co-rotating screws inside thebarrel having reverse flights at the second end beyond the outlet; and,means for rotating the screws.

The processes and apparatuses of the present invention advantageouslyreduce blockage in the discharge means, which reduces down timeassociated with such blockage. Furthermore, the processes andapparatuses of the present invention permit recovery of reactionmaterial and energy in the reaction system, thereby increasing the costeffectiveness of the process. Energy usage may be reduced by up to about30%, and possibly more, in the processes of the present invention.

In accordance with the present invention, an apparatus for producingisoolefin-diolefin rubbers includes a reaction vessel, a slurryprocessing vessel and a self-cleaning discharge means between thereaction vessel and the slurry processing vessel for transferring rubberslurry from the reaction vessel to the slurry processing vessel. Thereaction vessel, self-cleaning discharge means and slurry processingvessel may be integrated into an otherwise conventionalisoolefin-diolefin rubber process.

The reaction vessel may be any vessel suitable for the production,preferably continuous production of isoolefin-diolefin rubbers. Thereaction vessel must be able to maintain an appropriate reactiontemperature, typically in the range of from −110° C. to −50° C. Coolingis normally achieved by evaporating liquid ethylene external to thereaction vessel. Recycled cold liquid reaction medium may be used topre-cool feeds to the reaction vessel. Alternatively to, or inconjunction with pre-cooling feeds, recycled cold liquid reaction mediummay be used to assist in condensing compressed ethylene vapor bringingit closer to the reaction temperature. The reaction vessel typicallycomprises an outlet in fluid communication with a discharge means fordischarging rubber slurry out of the reaction vessel. Rubber slurryproduced in the reaction vessel comprises a high viscosity rubberportion and a lower viscosity liquid reaction medium portion.

In a first aspect, the function of the self-cleaning discharge means isto convey rubber slurry from the reaction vessel to the slurryprocessing vessel. The self-cleaning nature of the conveyor obviates theneed to employ other methods to prevent sticking of the rubber to theinside of the discharge means, for example, steam jacketing is notrequired.

In a second aspect, a discharge means may also act as an expressionseparator to separate the high viscosity rubber from at least some ofthe lower viscosity reaction medium. As is discussed below, separatingthe rubber from the reaction medium permits recovery of both thematerial of the reaction medium and the energy associated with the lowtemperature of the reaction medium.

A self-cleaning discharge means may be any suitable self-cleaning meansfor conveying rubber slurry from the reaction vessel to the slurryprocessing vessel. In one embodiment, the self-cleaning discharge meansis a self-cleaning screw conveyor, preferably a self-cleaning twin screwconveyor, more preferably a self-cleaning fully intermeshing twin screwconveyor, even more preferably a self-cleaning fully intermeshingco-rotating twin screw conveyor, and yet most preferably a self-cleaningfully intermeshing co-rotating twin screw extruder. Fully intermeshingco-rotating twin screws are generally more efficient in conveyingmaterial than fully intermeshing counter-rotation designs. In thelatter, rubber is subject to what is essentially a milling actionbetween the flight tip of one screw and the root of the other, leadingto high power usage and unnecessary working of the rubber.

In the case of a discharge means having a set of fully intermeshingco-rotating screws, for example, a self-cleaning fully intermeshingco-rotating twin screw extruder, the discharge means comprises a set offully intermeshed screws inside a barrel. The preferred arrangement isto have the discharge means operating in a reverse manner to that usedin a conventional extrusion process. The screws may be arranged to drawrubber slurry out of the reaction vessel, preferably at the top of thereaction vessel, through an inlet into the barrel and then to convey theslurry to the slurry processing vessel. Discharge to the slurryprocessing vessel may be accomplished by any suitable method, forexample, by having an outlet (or series of outlets) in the barrel,preferably pointing downwards into the slurry processing vessel.

Furthermore, seals for the screw shaft may be located on the oppositeside of the slurry processing vessel from the reaction vessel and ameans for rotating the screws (e.g. a drive comprising a motor andgearing) located beyond the seals. A reverse flighted screw section maybe situated beyond the outlet (or series of outlets) in the barrel toensure that slurry is discharged into the slurry processing vesselthrough the outlet (or series of outlets) and does not reach the seals.This arrangement allows the seals and the means for rotating the screwsto be located away from the low temperature reactor region, makingdesign and materials selection significantly simpler. A means foraccommodating thermal movement caused by the temperature differencebetween the cool reactor and warm slurry processing vessel may beinserted between the discharge means and the slurry processing vessel.Examples of such a means for accommodating thermal movement are asliding flange or a bellows joint. It will be evident to one skilled inthe art that other arrangements are also possible for the discharge ofrubber slurry from the screws into the slurry processing vessel.

The slurry processing vessel may be any vessel useful for furtherprocessing rubber slurry. In a butyl rubber plant, the slurry processingvessel may be a flash tank. In applications where the rubber is to behalogenated and the rubber has to be in solution form, the dischargemeans may be equipped with a solvent inlet so that the necessary solventcan be introduced to start the dissolution process. Furthermore,discharge would not be into a flash tank containing heated water, butinto a dissolution vessel that allows for dissolution of the rubber tobe completed and for residual volatile reaction medium to evaporate.

A discharge means may also act as an expression separator, particularlywhere recovery of cold liquid reaction medium is desirable. Preferably,such an expression separator is self-cleaning. Expression “is theseparation of liquid from a two-phase solid-liquid system by compressionunder conditions that permit the liquid to escape while the solid isretained between the compressing surfaces. Expression is distinguishedfrom filtration in that pressure is applied by movement of the retainingwalls instead of by pumping the material into a fixed space” (Perry'sChemical Engineers' Handbook, Sixth Edition, McGraw-Hill Inc. (1984) p.19-103). Any suitable expression separator may be used. Expression andexpression separators are described in Perry's Chemical Engineers'Handbook, Sixth Edition, McGraw-Hill Inc. (1984) pp. 19-103 to 19-107,the disclosure of which is herein incorporated by reference. Screwseparators are preferred, more preferably screw extruders, even morepreferably twin screw extruders. Particularly preferred is a fullyintermeshing twin screw extruder, for example a fully intermeshingco-rotating twin screw extruder or a fully intermeshing counter-rotatingtwin screw extruder. A fully intermeshing co-rotating twin screwextruder is more preferred, as it is more efficient, it consumes lesspower and it subjects the rubber to less milling action.

In the case where the expression separator is a fully intermeshingco-rotating twin screw extruder, the preferred arrangement is similarto, but more complex than, the geometry described above for the set offully intermeshing co-rotating screws used solely to convey rubber fromthe reaction vessel to the flash tank. When used as an expressionseparator, a fully intermeshing co-rotating twin screw extruder maycomprise a first section in which the screw geometry is designed toconvey the full flow of rubber slurry from the reaction vessel. Part wayalong the screws, the geometry may change so that the screws have onlysufficient capacity to convey the rubber portion, the rubber still beingswollen by absorbed liquid reaction medium. In this manner, the liquidreaction medium is expressed from the rubber slurry, thereby effectingseparation of at least part of the liquid reaction medium from therubber. Geometry changes may include, for example, changes to flightpitch, flight lead and land length, among others. The rubber portion isthen discharged into the slurry processing vessel, for example throughan outlet (or outlets) as described above.

A transport means in fluid communication with the expression separatorat a point (or points) between the reaction vessel and the slurryprocessing vessel may be used to transport separated liquid reactionmedium off stream from the expression separator. Transporting theseparated liquid reaction medium off stream provides a large variety ofopportunities for recovery and/or recycling. For example:

-   -   a. All or part of the liquid reaction medium may be recycled        back into the reaction vessel. By returning this cold medium to        the reaction vessel, the amount of fresh feed to the reaction        vessel is correspondingly reduced, therefore less energy is        needed to cool that feed. However, the fresh feed does need to        be higher in monomer level to compensate for depleted monomer        level in the reaction medium that is returned. Furthermore, the        load on the plant's vapor recycle system is greatly reduced,        particularly in view of the necessity to remove water from the        vapor being recycled from the flash tank.    -   b. All or part of the liquid reaction medium may be subjected to        a purification step before being recycled back into the reaction        vessel to reduce the likelihood of contamination and to reduce        accumulation of poisons to the polymerization reaction in the        reaction vessel. Purification may be accomplished, for example,        by filtration, by adsorbents (e.g. molecular sieves,        aluminosilicates), by a combination thereof, etc. Purification        of the liquid reaction medium may be conducted in order to, for        example, remove accumulated reaction by-products, separate        catalyst residues from the separation stream, etc. All or part        of the purified reaction medium may be recycled back into the        reaction vessel.

The energy of the liquid reaction medium may be recovered in a varietyof ways. For example:

-   -   a. The liquid reaction medium may be transported to a separate        heat exchanger for cooling feeds of one or more reaction        components, particularly for cooling the mixed feed.    -   b. The liquid reaction medium may be used to help cool the        reaction vessel, for example, by transporting it to a heat        exchanger to assist in condensing compressed reactor coolant        (e.g. ethylene vapor) bringing the coolant closer to the        reaction temperature.    -   c. The liquid reaction medium may be recycled back into the        reaction vessel, as mentioned above.    -   d. The transport means itself may be used as a heat exchanger        for cooling reactor feeds.        Any combination of the above may be employed and one skilled in        the art can easily determine other ways in which the separated        liquid reaction medium may be used to reduce the overall cost of        the process.

The transport means may be any suitable means for transporting separatedliquid reaction medium off stream. It may be a simple conduit, a conduitwith a filter, a pump, a combination thereof, etc. In one embodiment,the transport means is a mechanical filter. A mechanical filteradvantageously provides further separation of the liquid reaction mediumfrom the rubber portion of the rubber slurry, in addition to acting as atransport means for separated liquid reaction medium. In one embodiment,the mechanical filter is a screw conveyor, for example, a fullyintermeshing counter-rotating twin screw extruder or a fullyintermeshing co-rotating twin screw extruder. A mechanical filter iscapable of permitting passage of the liquid reaction medium through itwhile forcing rubber back into the expression separator. Preferably, thetransport means is a fully intermeshing counter-rotating twin screwextruder, wherein the screws operate to push the high viscosity rubberphase back into the expression separator while the clearance over thescrew flights and between the screws is such that lower viscosity liquidreaction medium is allowed to pass by.

In one embodiment, the slurry processing vessel is a flash tank (orflash drum as it may be called). A flash tank is generally a large tankhaving a pool of water, preferably hot water, at the bottom. The waterin the flash tank may be heated, for example, by steam. Rubber slurrydischarged from the discharge means falls into the water where thereaction medium and residual monomers are vaporized leaving the rubberin the form of a coarse slurry in hot water. If required, steam jets canbe used to break up the stream of rubber slurry as it emerges from thedischarge means. Rubber from the discharge means may be dischargedanywhere in the flash tank, although it is generally preferred todischarge the rubber near the middle of the lateral cross-sectional areaof the tank in order to reduce the chance of rubber sticking to thesides of the flash tank.

Isoolefin-diolefin rubbers are generally known in the art. Such rubbers,include, for example, butyl rubber (IIR), bromobutyl rubber (BIIR),chlorobutyl rubber (CIIR), among others. Butyl rubber may be formed bysuspension copolymerization of a monomer (e.g. isobutylene (97-99.5 wt%) with small amounts of isoprene (3.0-0.5 wt %)) in a liquidnon-aqueous reaction medium containing a catalyst. Other comonomers ortermonomers may be used to change the properties of the rubber. Thepolymerization reaction is conducted, preferably in a continuousreactor, at a temperature in a range of from about −110° C. to −50° C.,for example from about −110° C. to −70° C., or from about −100° C. to−90° C. Rubber formed in the reaction is generally insoluble in thereaction medium and forms a suspension (slurry) of fine particles in thereaction medium. While the rubber is said to be in solid or particulateform, one skilled in the art understands that such rubber solids orparticles are very soft, being swollen by absorbed liquids so that theyform a high viscosity phase. Halobutyl rubbers (e.g. chlorobutyl rubberand bromobutyl rubber) may be formed by halogenating butyl rubber in amanner generally known to one skilled in the art.

Any suitable liquid non-aqueous reaction medium may be used, forexample, alkyl halides, sulfuryl chloride, etc. Alkyl halides arepreferred, particularly methyl chloride. Any suitable catalyst may beused, for example Friedal-Craft's catalysts, syncatalysts, etc. Aluminumchloride is a preferred catalyst. The temperature is in a range of fromabout −110° C. to −50° C., for example from about −110° C. to −70° C.,or from about −100° C. to −90° C. The process is generally conducted ataround ambient pressure, e.g. about 0.7 atm to 1.2 atm.

The rubber content of the rubber slurry produced in the reaction vesselis ideally as high as possible to reduce the need to recycle reactionmedium. However, there is generally a practical limit to the rubbercontent of the slurry coming out of the reaction vessel. Generally, therubber content of the slurry is about 10%-40% by weight. Rubber contentmay be from about 20%-30% by weight or about 25% by weight. “Particle”size of the rubber may vary considerably, but is typically on the orderof about 20 microns. Agglomeration of particles may occur that increasesthe particles size, particularly during subsequent processing of theslurry.

When recycling is not done, rubber slurry produced in the reactionvessel is conveyed to a slurry processing zone by a discharge means asdescribed above. When recycling of liquid reaction medium and/or energyis done, rubber slurry exiting the reaction vessel enters aself-cleaning separation zone where the slurry is expressed to separateat least part of the liquid non-aqueous reaction medium from the rubber.The separated liquid reaction medium is transported off stream.Remaining slurry comprising residual liquid reaction medium and rubberare transported to the slurry processing zone. Examples of apparatusesfor expressing the rubber slurry and for transporting separated reactionmedium off stream have been previously described.

In one embodiment, the slurry processing zone may be in a flash tank. Ina flash tank, rubber slurry comes into contact with hot water and liquidreaction medium including unreacted monomers is vaporized, leaving therubber in the form of a coarse slurry in hot water. Small amounts ofantiagglomerate and stabilizer may be added to the rubber slurry at thisstage to prevent further agglomeration of the slurry particles and toprotect the rubber from degradation during further processing. A base,for example sodium hydroxide, may be added to neutralize catalystresidues. Vaporized reaction medium including unreacted monomers isremoved from the flash tank and then dried, compressed (cooled) andpurified before being recycled back into the reaction vessel.

Further processing and apparatus elements in an isoolefin-diolefinrubber process are conventional and are well known to one skilled in theart and may be used in conjunction with the inventive apparatuses andprocesses described herein.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be more clearly understood, preferredembodiments thereof will now be described in detail by way of example,with reference to the accompanying drawing in which:

FIG. 1 is a schematic diagram of a prior art process for production ofbutyl rubber;

FIG. 2 is a schematic diagram of a first embodiment of an apparatusaccording to the present invention in which a self-cleaning co-rotatingfully intermeshing twin screw extruder is used as a discharge systemfrom a butyl rubber reactor to a flash tank;

FIG. 3A is a plan view of a section of the screws of a self-cleaningfully intermeshing co-rotating twin screw extruder;

FIG. 3B is a series of end cross-sectional views of the screws of FIG.3A in a number of rotation positions;

FIG. 4 is a schematic diagram of a second embodiment of an apparatusaccording to the present invention in which a fully intermeshingcounter-rotating twin screw extruder is used as a mechanical filter inconjunction with a self-cleaning co-rotating fully intermeshing twinscrew extruder discharge system;

FIG. 5 is a schematic diagram of one embodiment of a process forproducing butyl rubber in accordance with the present invention in whichliquid reaction medium is recycled back into a reactor; and,

FIG. 6 is a schematic diagram of another embodiment of a process forproducing butyl rubber in accordance with the present invention in whichthe energy of liquid reaction medium is used to cool a mixed feedstream.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a prior art commercial process for total productionof butyl rubber is depicted. A feed stream of isobutylene and isoprenemonomers mixed in methyl chloride 2 and a feed stream of aluminumchloride catalyst dissolved in methyl chloride 4 are continuouslycharged to a butyl rubber reactor 6 at a temperature below about −90° C.Isobutylene and isoprene copolymerize at a temperature below about −90°C. in the presence of the catalyst to form a suspension (slurry) ofbutyl rubber in methyl chloride and residual monomers. The contents ofreactor 6 are cooled by evaporation of liquid ethylene in a coolingsystem 10. The slurry of butyl rubber in the reactor 6 is subjected tohigh flow velocities by an impeller 12 to reduce rubber build-up on theinner surfaces of the reactor. Rubber slurry having a rubberconcentration of about 25 wt % is discharged from the top of the reactor6 through a steam jacketed discharge line 14 into a flash tank 16. Atthe bottom of the flash tank 16 is a pool of hot water 18 at atemperature of about 75° C. continuously heated by a flow of steamthrough line 20. When cold rubber slurry contacts the hot water, methylchloride and residual isobutylene and isoprene are vaporized while thebutyl rubber forms a coarse slurry in hot water. Gaseous methylchloride, isobutylene and isoprene are drawn out the top of the flashtank 16 along with water vapor. The vapors are then subjected to anumber of processing steps 17 including drying, compressing andpurifying, leaving the bulk as a purified liquid that is recycledthrough line 19 into the feed stream 2. A stream 22 of antiagglomerate(0.4-1.0 wt % based on rubber of a mixture of stearic acid and zincstearate) and stabilizer (0.02-0.15 wt % alkylated diphenyl amine,alkylated phenyl amine, alkylated phenol or alkylated phenyl phosphite)are charged to the hot water in the flash tank 16. A stream of sodiumhydroxide 24 is charged to the hot water as well to neutralize catalystresidues. The coarse rubber slurry is separated from gross water 28 byscreening on a de-watering screen, and then fed successively into ade-watering extruder 30 and a drying extruder 32 where most of theremaining water is squeezed out. In some cases, one or more additionalstripper vessels may be used after the flash tank to further removeresidual volatile materials before the rubber is sent to the de-wateringscreen. On leaving the drying extruder 32, hot compressed rubber isexploded into a fluffy, porous crumb 34 by steam created from theresidual water, which has been heated under pressure. Crumb 34 is cooledand remaining water is allowed to evaporate on a conveyor 36. Crumb 34is then fed into balers 38 where it is compacted into bales, wrapped inpolyethylene wrapper and stacked for transport.

Referring to FIG. 2, a schematic diagram of a first embodiment of anapparatus according to the present invention is depicted. The apparatusprovides for simple discharge of rubber slurry from a butyl rubberreactor to a flash tank. A butyl rubber reactor 50 is in fluidcommunication with a self-cleaning fully intermeshing co-rotating twinscrew extruder 55, which includes a pair of screws 56, a barrel 57, anaperture 58 in the bottom of the barrel and a drive unit 59 comprising amotor and gearing for rotating the screws. The extruder 55 may be builtin sections to facilitate manufacture, and assembled into one unit. Theextruder 55 traverses a flash tank 60 such that the aperture 58 is nearthe middle of the flash tank. The extruder 55 is mounted on the flashtank 60 with bellows joints 63 a, 63 b acting to accommodate thermalmovement of the apparatus. Rotating shaft seals 62 (only one shown) foreach screw are situated where the screw shafts exits the flash tank, andserve to prevent the leakage of vapors or ingress of air. The closed topof the flash tank 60 includes a gas line outlet 61 through which gaseousmethyl chloride and gaseous residual isobutylene and isoprene monomersare drawn out of the flash tank. Rubber slurry is discharged from thetop of the reactor 50 and conveyed through the extruder 55 by a set offlights 65 pitched to permit displacement of the slurry in the extruder.Self-wiping action of the fully intermeshed co-rotating screws 56prevents rubber from sticking to surfaces inside the extruder, therebypreventing plugging of the extruder. Upon reaching the aperture 58,rubber slurry falls into the flash tank 60 and contacts hot water (notshown) at the bottom of the flash tank whereupon methyl chloride andresidual isobutylene and isoprene monomers are vaporized. To preventrubber slurry from passing by the aperture 58, the screws 56 comprise aset of reverse flights 66 beyond the aperture. The reverse flights 66are pitched to push rubber slurry back in the extruder and out throughthe aperture.

Reference is made to FIGS. 3A and 3B to further describe a fullyintermeshing co-rotating pair of screws. FIG. 3A is a plan view of asection of the screws in which rubber slurry is conveyed along theextruder. A first screw 100 includes one or more channels, one marked as102, and a first set of flights, one flight marked as 101. A secondscrew 120 includes one or more channels, one marked as 122, and a secondset of flights, one flight marked as 121. In this embodiment, atwo-start arrangement is shown. Similar action can be obtained withscrews having a single start or multiple starts greater than two.

The two screws intermesh so that as they rotate the tip of one flight onone of the screws wipes one of the channels of the other screw.Rotational movement of the two screws is synchronized by a geared driveto the screws. The two sets of flights fully intermesh so that the tipof one flight on one set of flights wipes against the face of theneighboring flight on the second set of flights. As the screws rotate,at a particular axial position, the wiping action alternates between thefirst and second sets of flights. This can be more clearly seen in FIG.3B, which is a series of cross-sectional end views of the twin screw ofFIG. 3A taken through the section A-A at a number of differentrotational positions, i.e. 0°, 15°, 33°, 45°, 57°, 65°, 90°, 105°, 125°,135°, 147°, 155° and 180°. In the following discussion, it will beevident that reference numerals labeled on any one rotational positionare also correspondingly applicable to the other rotational positions.

Still referring to FIG. 3B, it can be seen that the cross-section ofeach screw generally comprises three portions. Reference is made to thefirst screw 100, however, a similar description is also applicable tothe second screw 120. Referring to the 0° rotational position, the firstscrew 100 comprises two short cylindrical portions 104,105 that includesthe flights, then in the theoretically correct profile, two morecylindrical portions 106,107 that comprise the roots, and four generallyelliptical portions 108,109,110,111 that form the remainder of thechannels. In practice, for manufacturing convenience, the geometry doesvary from that which is theoretically correct but this is of littledetriment so long as the deviation is not large. Referring to the 33°rotational positions, there are four points of transition113,114,115,116 between the cylindrical flight portions and theelliptical portions. The direction of rotation of the screws isrepresented by the arrows within the two screws 100 and 120. Duringrotation from the 0° rotational position to the 90° rotational position,the lower flight of the first screw 100 wipes the channel of the secondscrew 120. The wiping action can be viewed as passing through threephases. In the first phase from 0° to 33°, the point 115 on the firstscrew scrapes the channel of the second screw. In the second phase from33° to 57°, the cylindrical portion 105 on the first screw scrapes theroot of the second screw. In the third phase from 57° to 90°, the point116 on the first screw scrapes the channel of the second screw.

It is evident that during rotation from the 90° rotational position tothe 180° rotational position, the upper flight on the second screw 120wipes the channel of the first screw 100 in a similar manner asdescribed above. It is also evident that rotation between 180° and 270°and between 270° and 360° follows a similar pattern. For example, duringthe rotation between 180° and 270°, the points 113,114 and thecylindrical portion 104 of the first screw 100 will wipe the channel ofthe second screw 120. Thus, the 360° rotational cycle of the screws maybe viewed as four 90° cycles in which the first and second screwsalternately wipe each other. The same type of self-wiping action may beobserved at any cross-section A-A taken along the length of the screws.

As indicated previously, flight tips are cylindrical in form. Theirdiameter is such as to provide a clearance inside the cylindricalportions of the barrel and sufficient width has to be allowed so thatadequate bearing surface is provided for successful mechanicaloperation.

Referring to FIG. 4, a schematic diagram of a second embodiment of anapparatus according to the present invention is depicted. The apparatusprovides for both discharge of rubber slurry from a butyl rubber reactorto a flash tank, and for separation of at least part of the liquidreaction medium from the rubber. A butyl rubber reactor 250 is in fluidcommunication with a self-cleaning fully intermeshing co-rotating twinscrew extruder 255, which includes a pair of screws 256, a barrel 257,an aperture 258 in the bottom of the barrel and a drive unit 259comprising a motor and gearing for rotating the screws. The extruder 255may be built in sections to facilitate manufacturing, and assembled intoone unit. The extruder 255 traverses a flash tank 260 such that theaperture 258 is near the middle of the flash tank. The extruder 255 ismounted on the flash tank 260 with bellows joints 263 a, 263 b acting toaccommodate thermal movement of the apparatus. Rotating shaft seals 262(only one shown) for each screw are situated where the screw shafts exitthe flash tank, and serve to prevent the leakage of vapors or ingress ofair. The closed top of the flash tank 260 includes a gas line outlet 261through which gaseous methyl chloride and gaseous residual isobutyleneand isoprene monomers are drawn out of the flash tank. A mechanicalfilter in the form of a fully intermeshing counter-rotating twin screwextruder 270 is in fluid communication with the extruder 255.

Rubber slurry is discharged from the top of the reactor 250 and conveyedthrough the extruder 255 by a set of flights 265 pitched to permitdisplacement of the slurry in the extruder and spaced to permitexpression of the slurry so that the high viscosity rubber phase isconveyed forward along the extruder while the low viscosity liquidreaction medium can leak back through the screw channels. The screwchannels of a fully intermeshing co-rotating twin screw extruderdescribe a helical flow path through which liquid reaction medium mayflow back through the extruder. The set of flights 265 may include afirst zone 267 in which the flights are spaced relatively widely apartto accommodate a relatively large volume of liquid reaction mediumpassing through the extruder 255. Farther along the extruder towards theaperture 258, the set of flights 265 comprises a second zone 268 inwhich the flights are spaced closer together. The change in flight pitchresults in a pressure gradient, which results in expression separationof the liquid reaction medium. In this manner, a separation is effectedbetween the rubber and the reaction medium in the extruder. Self-wipingaction of the fully intermeshed co-rotating screws 256 prevents rubberfrom sticking to surfaces inside the extruder, thereby preventingplugging of the extruder.

At a point along the first zone 267, a transport means embodied as amechanical filter in the form of a fully intermeshing counter-rotatingtwin screw extruder 270, for example the Mech Filt™ extruder fromNFM/Welding Engineers Inc (Massillon, Ohio, USA), is in fluidcommunication with the co-rotating twin screw extruder 255. Thecounter-rotating twin screw extruder 270 comprises a pair of screws 276,a barrel 277, an outlet 278 in the side of the barrel and a drive unit279 including a motor and gearing for rotating the screws. The screws ofthe counter-rotating twin screw extruder 270 are designed to push rubberback into the co-rotating twin screw extruder 255. However, since thecounter-rotating twin screw extruder 270 has narrow clearances betweenthe screws and between the flights of the screws and the barrel, lowviscosity liquid is able to pass up through the extruder 270, whereasthe rubber is not. The rubber is thereby retained within extruder 255and not allowed to exit through the extruder 270. The separated liquidreaction medium is transported off stream and discharged from thecounter-rotating twin screw extruder 270 via the outlet 278.

Rubber and residual liquid reaction medium which continued through theco-rotating twin screw extruder 255 eventually reaches the aperture 258,falls into the flash tank 260 and contacts hot water (not shown) at thebottom of the flash tank whereupon methyl chloride and residualisobutylene and isoprene monomers are vaporized. To prevent rubber frompassing by the aperture 258, the screws 256 include a set of reverseflights 266 beyond the aperture. The reverse flights 266 are designed topush rubber slurry back in the extruder 255 and out through the aperture258.

FIG. 5 depicts one embodiment of a process for producing butyl rubber inaccordance with the present invention in which liquid reaction medium isrecycled back into a butyl rubber reactor. Fresh feeds 300 together withrecycled materials 318 go into a storage and blending system 320, whichoperates at close to ambient temperature. A mixed feed 301, includingisobutylene and isoprene monomers in liquid methyl chloride inproportions required by the process, is transported from the storage andblending system 320 to a feed cooler 321 where the mixed feed is cooledto below about −90° C. After cooling, the mixed feed is joined by coldseparated liquid reaction medium from line 312 to form a combined feedstream 302. The cold combined feed stream 302 and a cold (below about−90° C.) catalyst feed stream 303 of aluminum chloride catalyst inliquid methyl chloride are continuously fed into a reactor 305 wherepolymerization occurs at a temperature normally maintained in a rangefrom about −90° C. to about −98° C. by a cooling system 306 utilizingthe evaporation of liquid ethylene. Rubber slurry produced duringpolymerization is transported to a self-cleaning co-rotating fullyintermeshing twin screw extruder 310 where rubber phase is separated byexpression from the liquid reaction medium. Residual methyl chloride andmonomers are flashed from the rubber by hot water at a temperature ofabout 75° C. in a flash tank 315. Methyl chloride and monomer vapors aretransported through line 316 and subjected to a number of processingsteps 317, including drying, compressing and purifying, leaving the bulkas a purified liquid that is recycled through line 318 into the storageand blending system 320 where it is mixed with the fresh feeds 300. Coldseparated liquid reaction medium is transported off stream from theco-rotating fully intermeshing twin screw extruder 310 by acounter-rotating fully intermeshing twin screw extruder 311, thentransported through line 312 to join the mixed feed forming the combinedfeed stream 302 that enters the reactor 305. Recycling of the coldliquid reaction medium results in less energy being used to cool thereduced quantity of mixed feed. Other features of the process are knownto one skilled in the art.

FIG. 6 depicts another embodiment of a process for producing butylrubber in accordance with the present invention in which the energy ofcold liquid reaction medium is used to cool a mixed feed of methylchloride and monomers. Fresh feeds 400 together with recycled materials418 go into a storage and blending system 420, which operates at closeto ambient temperature. A mixed feed 401, including isobutylene andisoprene monomers in liquid methyl chloride in proportions required bythe process, is transported from the storage and blending system 420 toa preliminary heat exchanger 422 and then to a final cooler 421 wherethe mixed feed is cooled to below about −90° C. to form a cold mixedfeed stream 402. The cold mixed feed stream 402 and a cold (below about−90° C.) catalyst feed stream 403 of aluminum chloride catalyst inmethyl chloride are continuously fed into a reactor 405 wherepolymerization occurs at a temperature normally maintained in a rangefrom about −90° C. to about −98° C. by a cooling system 406 utilizingthe evaporation of liquid ethylene. Rubber slurry produced duringpolymerization is transported to a self-cleaning co-rotating fullyintermeshing twin screw extruder 410 where rubber phase is separated byexpression from the liquid reaction medium. Residual methyl chloride andmonomers are flashed from the rubber by hot water at a temperature ofabout 75° C. in a flash tank 415. Methyl chloride and monomer vapors aretransported through line 416 and subjected to a number of processingsteps 417, including drying, compressing and purifying, leaving the bulkas a purified liquid that is recycled through line 418 into the storageand blending system 420. Cold separated liquid reaction medium istransported off stream from the co-rotating fully intermeshing twinscrew extruder 410 by a counter-rotating fully intermeshing twin screwextruder 411 and transported through line 412 to the preliminary heatexchanger 422 to provide substantial cooling of the mixed feed. In thisway, less energy is required by a heat exchanger, using liquid ethylene,as a final cooler 421 to cool the mixed feed before it enters thereactor 405. After being used to help cool the mixed feed, the separatedliquid reaction medium is transported through line 423 to the flash tankwhere volatile components (e.g. methyl chloride and monomers) arevaporized and recycled as described above, while non-volatile components(e.g. oligomers and low molecular weight polymer components) become partof the rubber. Other features of the process are known to one skilled inthe art.

The processes of the present invention may be applied on a commercial orpilot scale; however, certain process conditions may differ between acommercial and a pilot scale process. On a commercial scale, the flowthrough in the reactor may be on the order of 12,000 kg/h while on apilot scale, the flow through may be about 8 kg/h. In the mixed feed,the ratio of methyl chloride to monomers may be about 69:31 on acommercial scale, while on a pilot scale the ratio may be about 82:18.Discharge concentrations of methyl chloride, residual monomers andrubber from the reactor on a commercial scale may be in a ratio of about69:6:25 respectively, while in a pilot scale reactor the ratio may beabout 82:6:12 respectively.

Where liquid reaction medium is being separated from rubber in anexpression separator (e.g. a fully intermeshing co-rotating twin screwextruder), the concentration of solids being discharged from theextruder can be about 68 wt % on both commercial and pilot scales.Therefore, on a commercial scale, the amount of liquid separated forrecycling may be about 84%, while on a pilot scale, the amount of liquidbeing separated for recycling may be about 94%. Given that the amount ofliquid separated for recycling may be about 84% on a commercial scale,the total heat removed by refrigeration in a commercial process of thepresent invention would be about 70% of that in a conventionalcommercial process. In the case of the embodiment shown in FIG. 5,further economies result from the reduced quantity of vaporized reactionmedium recycled from the flash tank.

A fully intermeshing co-rotating twin screw extruder particularly usefulon a pilot scale may have a 2 start design having a flight lead of about38 mm, a screw diameter of about 24 mm, an extruder length of about 36diameters and a rotational speed of about 100 rpm.

Having thus specifically described the present invention, it will beevident to one skilled in the art that modifications may be made whichare encompassed by the scope of the invention claimed hereafter.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

1. An apparatus for transferring an isoolefin-diolefin rubber slurryhaving a temperature in a range of from −110° C. to −50° C. from areaction vessel to a slurry processing vessel comprising: (a) a barrelhaving a first end and a second end; (b) an inlet proximal the first endfor receiving the rubber into the barrel; (c) an outlet proximal thesecond end for discharging the rubber from the barrel; (d) a set offully intermeshing, co-rotating screws inside the barrel having reverseflights at the second end beyond the outlet; (e) motor and gearing forrotating the screws; and (f) a conduit in fluid communication with thebarrel at one or more points for transporting the liquid non-aqueousreaction medium separated from a slurry away from the reaction vessel.2. The apparatus according to claim 1, wherein the set of screws is afully intermeshing co-rotating pair of screws.
 3. An apparatus forproducing an isoolefin-diolefin rubber comprising: (a) a reaction vesselfor preparing a slurry of an isoolefin-diolefin rubber in a liquidnon-aqueous reaction medium at a temperature in a range of from −110° C.to −50° C.; (b) an expression separator in fluid communication with thereaction vessel for separating at least part of the liquid non-aqueousreaction medium in the liquid form from the rubber, the expressionseparator comprising: (i) a barrel having a first end and a second end;(ii) an inlet proximal the first end for receiving the rubber into thebarrel from the reaction vessel; (iii) an outlet proximal the second endfor discharging the rubber from the barrel; (iv) a set of fullyintermeshing, co-rotating screws inside the barrel having reverseflights at the second end beyond the outlet; (v) motor and gearing forrotating the screws; (c) a slurry processing vessel in fluidcommunication with the expression separator for receiving residualslurry from the expression separator; and, (d) a conduit in fluidcommunication with the expression separator at one or more pointsbetween the reaction vessel and the slurry processing vessel fortransporting liquid non-aqueous reaction medium separated in liquid formfrom the rubber by the expression separator out of the expressionseparator and away from the reaction vessel.